Stan Simpson, a chemist and past Director of the Wool Research
Organisation, Lincoln, responded.

All animals, including humans, have only two forms of coloured cells
in their hair. Melanin is the chemical name for the material which
fills either a yellow or brown-black type of granule. There is no
precise size to the melanin molecules, as they appear to form polymers
from complex molecules (called quinones) in the hair fibre as it grows.

Melanins absorb selective colours of sunlight. The reflected light
remaining after absorption is what we see as fair, brown, grey or black
hair, the latter involving complete absorption of sunlight. Almost all
of the packets of light energy merely heat the hair, but occasionally
a quantum of light energy will break a weak chemical bond and hence the
molecule. Lots of types of these photochemical reactions are well known.
The effect is to gradually destroy the light-absorbing chemical structure
of melanin, and as a result our hair is progressively bleached.

Sunlight also slowly destroys synthetic dye colours used in textiles and
in industry a number grading system is used to describe the resistance
of the dye to fading. Melanins fall in the middle of the range, although
the effects of sunlight bleaching are noticed more readily with lightly
coloured hair.